CONSTRUCTION MANAGEMENT OF A MECHANICALLY … · CONSTRUCTION MANAGEMENT OF A MECHANICALLY DREDGED,...

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CONSTRUCTION MANAGEMENT OF A MECHANICALLY DREDGED, IMPACTED SEDIMENT PROJECT T. Merritts 1 , W. Dinicola 2 , and W. C. Leuteritz, P.E., LSP 3 ABSTRACT An area of 0.55 hectares (1.35 acres) within a river in the northeast United States was deemed to be in a condition of Readily Apparent Harm due to the visibility of oil-like material/tar-like material. Removal of the impacted sediments through the use of a mechanical dredge was determined to be the most appropriate remedial action. In order for equipment and materials to access the river, an access ramp was constructed in the riverbank spanning from the shoreline to the construction staging area (parking lot of a former food-processing facility being rented by the Contractor). A long-reach excavator positioned on a barge was deployed to dredge the sediments which were then transported back to the staging area by a scow. Dredged materials were then stored in the parking lot area to dewater until they were of acceptable water content to be trucked to an off-site disposal facility. Backfilling operations were conducted similarly, as materials were trucked to the site, stored in the parking lot area, and then transported to the excavator by either barge or scow to be placed in the river. Approximately 6,048 m 3 (7,910 cubic yards) of coal tar impacted sediments were mechanically dredged from the river. Additionally, 6,330 tonnes (6,978 tons) of backfill, consisting of a filter layer and riprap layer, was placed to protect the newly exposed riverbed. Both the HYPACK software installed on the long-reach excavator and the single-beam bathymetric surveys performed by a subcontractor were employed over the course of the project to ensure proper material removal/placement depths had been obtained. Restoration activities resulted in the Site being left in conditions similar to those existing prior to construction and also included extensive replanting activities. Keywords: Dredging, Construction Management, Oil-like Material, River INTRODUCTION The site is a former manufactured gas plant situated on 4.45 hectares (11 acres) of land that was being used as an active gas distribution and operations center by KeySpan in Manchester, New Hampshire. The area of impacted sediments was determined to be 5,481 square meters (1.35 acres) of river bottom adjacent to the eastern bank of the Merrimack River. The dredge prism, which was designed based on the compilation of results of all sampling efforts performed at the site to capture the spatial and vertical extent of impacted sediments, dictated the removal of 5,791 m 3 (7,574 cubic yards) of material from the river. Construction consisted of primarily two tasks: impacted sediment removal and backfill placement. The impacted sediments were removed mechanically, transported to a nearshore dredged material offloading area, and then transferred to sediment dewatering area (a former food-processing facility parking lot beneath the Queen City Bridge) for dewatering. Once dewatered, sediments were transported via truck to an off-site disposal facility for thermal treatment. Water collected during the dewatering process was treated on-site and discharged to the city sewer system. Backfill materials were delivered to the site via truck and placed mechanically, utilizing the same equipment used for sediment removal (after undergoing a thorough decontamination process). The dredged material offloading area consisted of a ramp that was constructed to span between the eastern bank of the river and the staging and laydown area. The staging and laydown area was also located in the parking lot area of 1 T. Merritts, EIT, Anchor Environmental, L.L.C., 9194 Red Branch Road, Suite B, Columbia, MD 21045, T: 410- 715-0824, Fax: 410-715-5681, Email: [email protected] 2 W. Dinicola, Anchor Environmental, L.L.C., 9194 Red Branch Road, Suite B, Columbia, MD 21045, T: 410-715- 0824, Fax: 410-715-5681, Email: [email protected] 3 W. Christian Leuteritz, P.E., L.S.P., Anchor Environmental, L.L.C., 10 New England Business Center Drive, Suite 102, Andover, MA, 01810, T: (978) 974-9090, Fax: (978) 974-9091, Email: [email protected]. 351

Transcript of CONSTRUCTION MANAGEMENT OF A MECHANICALLY … · CONSTRUCTION MANAGEMENT OF A MECHANICALLY DREDGED,...

CONSTRUCTION MANAGEMENT OF A MECHANICALLY DREDGED, IMPACTED SEDIMENT PROJECT

T. Merritts1, W. Dinicola2, and W. C. Leuteritz, P.E., LSP3

ABSTRACT

An area of 0.55 hectares (1.35 acres) within a river in the northeast United States was deemed to be in a condition of Readily Apparent Harm due to the visibility of oil-like material/tar-like material. Removal of the impacted sediments through the use of a mechanical dredge was determined to be the most appropriate remedial action. In order for equipment and materials to access the river, an access ramp was constructed in the riverbank spanning from the shoreline to the construction staging area (parking lot of a former food-processing facility being rented by the Contractor). A long-reach excavator positioned on a barge was deployed to dredge the sediments which were then transported back to the staging area by a scow. Dredged materials were then stored in the parking lot area to dewater until they were of acceptable water content to be trucked to an off-site disposal facility. Backfilling operations were conducted similarly, as materials were trucked to the site, stored in the parking lot area, and then transported to the excavator by either barge or scow to be placed in the river.

Approximately 6,048 m3 (7,910 cubic yards) of coal tar impacted sediments were mechanically dredged from the river. Additionally, 6,330 tonnes (6,978 tons) of backfill, consisting of a filter layer and riprap layer, was placed to protect the newly exposed riverbed. Both the HYPACK software installed on the long-reach excavator and the single-beam bathymetric surveys performed by a subcontractor were employed over the course of the project to ensure proper material removal/placement depths had been obtained. Restoration activities resulted in the Site being left in conditions similar to those existing prior to construction and also included extensive replanting activities.

Keywords: Dredging, Construction Management, Oil-like Material, River

INTRODUCTION

The site is a former manufactured gas plant situated on 4.45 hectares (11 acres) of land that was being used as an active gas distribution and operations center by KeySpan in Manchester, New Hampshire. The area of impacted sediments was determined to be 5,481 square meters (1.35 acres) of river bottom adjacent to the eastern bank of the Merrimack River. The dredge prism, which was designed based on the compilation of results of all sampling efforts performed at the site to capture the spatial and vertical extent of impacted sediments, dictated the removal of 5,791 m3 (7,574 cubic yards) of material from the river.

Construction consisted of primarily two tasks: impacted sediment removal and backfill placement. The impacted sediments were removed mechanically, transported to a nearshore dredged material offloading area, and then transferred to sediment dewatering area (a former food-processing facility parking lot beneath the Queen City Bridge) for dewatering. Once dewatered, sediments were transported via truck to an off-site disposal facility for thermal treatment. Water collected during the dewatering process was treated on-site and discharged to the city sewer system. Backfill materials were delivered to the site via truck and placed mechanically, utilizing the same equipment used for sediment removal (after undergoing a thorough decontamination process).

The dredged material offloading area consisted of a ramp that was constructed to span between the eastern bank of the river and the staging and laydown area. The staging and laydown area was also located in the parking lot area of

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1 T. Merritts, EIT, Anchor Environmental, L.L.C., 9194 Red Branch Road, Suite B, Columbia, MD 21045, T: 410-715-0824, Fax: 410-715-5681, Email: [email protected]

2 W. Dinicola, Anchor Environmental, L.L.C., 9194 Red Branch Road, Suite B, Columbia, MD 21045, T: 410-715-0824, Fax: 410-715-5681, Email: [email protected]

3 W. Christian Leuteritz, P.E., L.S.P., Anchor Environmental, L.L.C., 10 New England Business Center Drive, Suite 102, Andover, MA, 01810, T: (978) 974-9090, Fax: (978) 974-9091, Email: [email protected].

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the former food-processing facility. To facilitate offloading of dredged material, a 12.2 m (40 foot) by 12.2 m (40 foot) temporary floating dock was placed in the river just north of the Queen City Bridge from which a long-reach excavator would transfer material from the river to an on-shore material transfer area (and vice versa during backfilling operations). An access ramp was then constructed between the floating dock and the sediment staging area to allow transport of dredged material from the material transfer area to the sediment dewatering area. The site plan for the offloading area is shown on Figure 1. The floating dock and access ramp transition is shown on Figure 2. The ramp was constructed immediately adjacent to the Queen City Bridge, which allowed for construction of the ramp without removing any significantly large trees along the bank of the river; however, some smaller shrubs and bushes were removed. This area was replanted and restored to pre-project conditions set forth in the Remedial Design Report (RDR; Anchor 2007) upon completion of the work.

Figure 1 - Site staging area

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Figure 2 - Access ramp and floating dock details

The access ramp constructed from the parking lot area of the former food-processing facility to the floating dock stationed on the eastern shore of the river crossed a portion of the existing River Walk, an asphalt walking path located along the eastern shore of the river. Both ends of the removed River Walk path section were saw-cut where they overlapped the access area. Removed pavement from the walkway was stored in the parking lot area before being transported to ESMI for disposal. Clean soil excavated from the riverbank to construct the access ramp was stockpiled in the sediment staging area for use during site restoration. The access ramp was graded to a 4 horizontal to 1 vertical (4H:1V) slope from the river edge to the former food-processing facility (see Figure 2). As noted on Figure 2, an asphalt berm was installed at the top of the ramp to prevent storm water from flowing from the staging area pavement down the ramp and ultimately, to the river. The ramp was covered with 10.2 cm (4 inches) of 3.81 cm (1-1/2-inch) crushed gravel to prevent erosion of the ramp surface. As necessary, a layer of clean gravel was placed on the access ramp to limit the tracking of sediment from the sediment transfer area to the parking lot and vice versa. The side slopes of the access ramp were secured utilizing polyethylene sheeting, sand bags, and wooden stakes. Additionally, hay bales and silt fencing were installed along the top of the side slopes. Once per week, or immediately following a rainfall of more than 2.5cm (1 inch), a Storm Water Pollution Prevention Plan (SWPPP)

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investigation was conducted. The purpose of the SWPPP was to identify any access ramp controls that required repairs or adjustments to prevent site storm water runoff from reaching the river.

The remedial action goals for the site included: (1) provide protection for the environment by mitigating the condition of Readily Apparent Harm associated with the visible oil-like material observed in the river sediments, (2) eliminate further discharge from the visually impacted material within the designed dredge prism, and (3) protect the existing flora (protected vegetation) to the extent practicable.

Barges and work boats were launched from the public boat ramp, located approximately 915 m (1,000 yards) upstream of the dredged material offloading area, and floated via a push boat for setup of the perimeter silt curtain and oil boom system.

OBSERVATIONS DURING MATERIAL REMOVAL

In order to determine the volume of material necessary to be removed from the river to satisfy the previously described remedial goals for the site, a dredge prism was developed utilizing the Thiessen polygon method. This method assumes a constant depth of impact throughout a certain area, or polygon, which is determined by analyzing core samples collected from the river. The following assumptions were made to generate the dredge prism surfaces used to estimate dredge volumes:

� A 2H:1V slope was applied along the perimeter of the dredge area to ensure slope stability of the riverbed and embankment after completion of dredging.

� Vertical slopes were assumed at the interface between individual dredge areas (i.e., polygons). Although not incorporated in the 3-D dredge surface, an internal slope of 2H:1V was applied to dredge transition areas where the difference in adjacent dredge cuts exceeded 0.762 m (2.5 feet). This was done to ensure slope stability of post-dredged areas and to prevent sloughing of material between adjacent dredge polygons.

� The interpreted depths of impact noted within two hardened tar deposit areas (identified during site investigations and field sampling efforts) were estimated based on visual observation during low water conditions. These depths were subject to change and were adjusted as necessary based on observations in the field.

Based on the surfaces generated using the assumptions described above, the proposed total neatline dredge volume was determined to be approximately 5,791 m3 (7,574 cy). When a 0.15 cm (6 inch) overdredge allowance was incorporated into the volume, the total dredge volume increased to approximately 6,737 m3 (8,811 cy). An additional volume of approximately 109 m3 (143 cy) was added to the estimated total dredge volume to account for internal side slopes between dredge polygons with a greater than 0.31 m (1 foot) difference in dredge cut depth.

Because the remedial action cleanup goal was to remove visually impacted shallow sediments in the river, the dredge prism was developed based on visual observations from the series of investigations performed in the river and should not be considered a chemical-concentration-based remedy. As such, no post-dredge sampling was performed at the site. Success of the remedy was to be primarily based on the map of dredge cuts produced by the HYPACK software employed on the dredge and verified by removal of an anticipated dredge volume that includes an allowable 0.15 m (6 inch) overdredge depth.

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Table 1 - Thiessen Polygon Areas

Location ID Polygon Area

(square meters) Location ID Polygon Area (square feet)

TB-01 88.8 MRRV-15B 858 RB-23 66.1 MRRV-16A 856 RB-24 222.1 MRRV-17A 1,293 RB-25 70.1 MRRV-18A 144.3 RB-26 111.0 MRRV-18B 49.5 RB-27 353.0 MRRV-19A 104.3 RB-28 366.9 MRRV-20A 137.5 RB-30 167.9 MRRV-21A 162.0 RB-32 147.3 MRRV-22A 146.1 RB-33 154.3 MRRV-23A 128.8

MRRD-01 62.9 MRRV-23B 79.7 MRRG-04 191.0 MRRV-24A 140.3 MRRG-06 102.7 MRRV-25A 146.4 MRRG-08 84.3 MRRV-26A 126.1 MRRG-09 61.7 MRRV-26B 147.2 MRRG-10 100.8 MRRV-27A 103.3 MRRG-12 107.0 MRRV-28A 212.0 MRRG-13 131.6 MRRV-29A 278.9 MRRG-14 151.1 MRRV-30A 260.8

Impacted sediment was removed utilizing mechanical dredging at this site due to the hardened, cohesive nature of the impacted sediments. Dredging occurred only within areas that utilized a silt curtain and oil boom system to protect the surrounding water from residual deposition and high turbidity.

A hydraulically-actuated 1.72 m3 (2-1/4 cy) clamshell bucket mounted on the excavator was used to dredge the sediments. When fully opened, the bucket was capable of taking a 1.83 m (6 foot) by 1.98 m (6.5 foot) bite with an average depth of 0.61 m (2 feet) across the bite. The bucket was considered a level-cut bucket, meaning the bottom of the dredge cut was not scalloped, as would be the case with a traditional clamshell bucket. This feature of the bucket reduced the need for cut overlap and overdredging to achieve the designed cut surface. The bucket was equipped with a HYPACK global positioning system (GPS) to allow for precise control of dredging operations. The HYPACK software package displayed color-coded depth information in both plan and sectional views to allow the operator to view as-dredged depths in real time. The screen in the equipment cab enabled the operator to see precisely where the bucket was located relative to previous bucket cuts. This control ensured that complete coverage was achieved. Additionally, the operator could see the bottom elevation of the bucket on the screen, thus allowing him to control the depth of cut with relatively precise accuracy. If a cut of less than 0.61 m (2 feet) was required at a location, the operator could control the vertical location of the bucket to remove less than 0.61 m (2 feet) of material. If an area required a 0.61 m (2 foot) cut, the bucket was pushed into the sediments and closed. If a cut of more than 0.61 m (2 feet) was required, multiple cuts were taken until the design depth was achieved.

The dredge platform was secured in place through the use of spud anchors. The spuds were pushed into the river bottom using the excavator bucket in preparation for dredging operations. To change dredge location once a certain area had been dredged to the appropriate design depth, the spuds were lifted one at a time using the excavator bucket, and the dredge platform was allowed to float downstream to the next location. In order to move east or west, the spuds were lifted using the excavator bucket, and the transport scow was used to “spin” the barge into the required location.

Throughout dredging operations, significantly large debris was encountered and removed from the river. Typical debris removed included large rocks, tires, trees, and various large pieces of scrap metal. On a few occasions, encountered debris was too large to be removed with the on-site equipment, and in these situations, the debris was

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characterized, marked by GPS coordinates, and left in place. Dredged debris was transported alongside removed impacted sediment via scow along the eastern half of the river to the floating dock at the shoreline adjacent to the former food-processing facility. These scows were then offloaded as previously described. Wooden debris was separated from the dredged sediment and other debris and shipped to a separate disposal facility for final disposal.

Dredging operations commenced on July 12, 2007, and ended on October 17, 2007. A total of 6,048 m3 (7,910 cy) of sediments, as determined by barge loads removed from the river and verified by weight tickets of trucks delivering dredged sediment to the off-site disposal facility, were dredged from the river and shoreline. This volume equates to approximately 96 percent of the design volume. Certain areas in the dredge prism were dredged to a deeper or shallower depth than was originally designed due to the presence of unmovable debris or for sediment characterization reasons outlined in the following section.

Deviations from Dredging Design

In three locations within the dredge prism, the design depth required the dredge to cut past the impacted sediments and into a thick, clay till layer. This mischaracterization of sediment layers was likely due to poor recovery during previous sediment coring activities, which provided only a rough characterization of the vertical extent of sediments within a certain polygon. The clay till layer proved difficult to cut into and produced a heavy turbidity plume too great to be contained by the turbidity curtain/oil boom system that was in place. This plume resulted in turbidity measurements that exceeded the action limits established in the construction permits. Typically, measures were taken to decrease the turbidity in the area, such as temporarily suspending dredging operations, relocating or adding silt curtains to the area, and decreasing the production rate of the dredge to reduce the number of bucket cuts per minute. However, in some areas it was determined that dredging could not continue at any rate without producing a turbidity exceedance. In these areas, sample dredge cuts were pulled from the till layer and examined on the dredge barge deck. If the material was determined to be free of impacted sediments, it was photographed, marked in the appropriate field books, and the dredge cut depth was revised. The dredge cut depths revisions varied from 0.15 m (0.5 feet) to 0.61 m (2.0 feet).

In dredge areas very near to the shoreline and in the area directly underneath the railroad bridge, the GPS HYPACK system was unable to receive adequate satellite signal strength to make the system effective. The poor satellite coverage in these areas was due to either an overhead railroad bridge that spanned above the dredge prism or overhanging trees along the shoreline. In lieu of GPS data in these areas, 0.31 m (1.0 foot) interval markings were placed on the excavator boom in a manner so that they were visible at all times by the operator. The operator would then use these markings to determine the relative depth of the existing river bottom by placing the bucket on the river bottom and noting the boom marking at the water surface. He would then work off of this boom marking, removing material until the design depth in a given polygon had been met. In these areas, the boom markings were recorded in the appropriate field books and, upon completion of dredging, used to supplement the HYPACK data to verify that the appropriate dredge cut depths had been met.

Material from the two hardened tar deposits (see Figure 3) was removed from the river bottom and shoreline using the same equipment and procedures used to perform all other site dredging. As described in the RDR (Anchor 2007), the technical practicability of removing all of this material was assessed. The equipment utilized at the site was able to remove the hardened tar material that was located in the river. However, in both the northern and southern areas, a thin tar layer extended back into the river embankment. These impacted sediments extended beyond the dredging limits and into areas where removal of the sediments would also require the removal of protected vegetation (mature silver maples). The newly exposed shoreline faces in these areas were producing a heavy oil-like sheen in the river, which was then captured by the oil boom system. It was determined that the best course of action for neutralizing the impact of these sediments to the river was to place an organoclay reactive mat over the shoreline in these areas and anchor the mat in place using the stone armoring as designed for use in the backfilling operations in the surrounding river areas. A letter was sent to the New Hampshire Department of Environmental Services (NHDES) requesting approval of this design modification. Approval for this design modification was granted by NHDES on October 30, 2007. A total of 92.9 m2 (1,000 square feet) of organoclay matting was placed and anchored in these two areas (see Figure 3). The matting provided an immediate elimination of the oil-like sheen emanating from the shoreline.

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Figure 3 – Organoclay mat placement locations

SEDIMENT DEWATERING AND WATER TREATMENT

After removal of impacted sediment from the river and transport to the nearshore offloading area, the sediment was transferred to the sediment dewatering area located in the parking lot of the former food-processing facility. The dredged material was transferred from the offloading area utilizing a long-reach excavator operating from the floating dock at the toe of the access ramp. The long-reach excavator transferred the material to a metal material hopper that had been secured on the access ramp. The front-end loader then moved the material from the metal hopper to the sediment dewatering area. Once in the sediment dewatering area, material was stockpiled and allowed to gravity drain of any excess water present in the sediments (see Figure 4).

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The stockpile was located in an area of the parking lot that took advantage of existing slopes to facilitate the flow of free water to the southwest corner of the dewatering area. The water was then pumped to the on-site water treatment tanks shown on Figure 1. The pump located in the sediment dewatering area was surrounded by gravel, which served as a filter to remove any fine sediment particles from the water. This gravel would, as anticipated, become clogged with fine sediments and was replaced as necessary (typically 1 to 2 times per week). Discarded gravel was then transported off-site for final disposal.

�Figure 4 - Sediment dewatering area

In order to prevent free water or storm water runoff from the stockpile from spreading to any other areas on site including the river, controls were set in place to isolate the stockpile. An approximately 0.61 m (2 foot) high clean sand berm was constructed around the perimeter of the dewatering area and was covered with polyethylene sheeting. Additionally, hay bales were placed so that they formed a barrier outside of the clean sand berm. At the conclusion of site activities every day, the sediment stockpile was covered with tarps to eliminate the need for site air monitoring and to reduce the possibility of any odor/human contact concerns.

Once sediments were dry enough to pass a standard paint filter test, they were transported via truck to an off-site sediment disposal facility and thermally treated. According to the project Bills of Lading, a total of 10,315 tonnes (11,370 tons) of material were shipped off site throughout construction.

Collected water from dewatering operations was pumped to the on-site water treatment system which consisted of two water fractionation tanks that utilized sediment settlement methodology. Following treatment, the water was discharged to the City of Manchester sewer system located adjacent to the former food-processing facility. Under the conditions of the City of Manchester permit, no more than 18.93 m3 (5,000 gallons) per day of treated water were allowed to be discharged to city sewer system. Over the course of the project, a total of 702.9 m3 (185,700 gallons) of water were treated and discharged.

OBSERVATIONS DURING BACKFILL MATERIAL PLACEMENT

Given the configuration of the river and the need to stabilize the river bottom and eastern river embankment, an armor layer was placed over the dredged area. This armor layer was designed to withstand the high current velocities generated by this area of the river, especially during the spring runoff and flood conditions. In addition, the shoreline adjacent to the dredge area was stabilized with placement of riprap.

The riprap protection was designed to resist erosive forces associated with the 100-year flood event as determined by the hydrodynamic modeling conducted for the site, discussed in the RDR (Anchor 2007). The dredge area was

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divided into three main zones based on the distribution of the flow velocity and the 100-year flow depth predicted by the hydrodynamic model.

Immediately following completion of dredging, the newly exposed riverbed was protected by the placement of filter and riprap material over the dredged area. The protection extended upslope on the east bank to an approximate elevation of 117 feet (North American Vertical Datum [NAVD] 88), corresponding to the bottom of the exposed protected vegetation root zone. The riprap was designed according to the U.S. Army Corps of Engineers (Corps) Engineering Design Manual, Hydraulic Design of Flood Control Channels (Corps 1991). The Corps methodology uses a depth-averaged local velocity to calculate the corresponding representative stone size, D30 (i.e., riprap size, of which 30 percent is finer by weight), required to resist this velocity. D30 is proportional to the local depth-averaged velocity and inversely proportional to the local depth of flow. D30 also depends on the rock size, vertical variation of velocity (i.e., along the water column), bank side slope, and location of the riprap within the river.

Table 2 - Backfill Design Volumes by Zone

Zone Minimum Layer Thickness

Minimum Volumes Filter Layer

(m3)Armor Layer

(m3)

Zone 1 Armor Layer Thickness = 0.61 m (24”) Filter Layer Thickness = 0.15 m (6”) 209 834

Zone 2 Armor Layer Thickness = 0.31 m (12”) Filter Layer Thickness = 0.15 m (6”) 578 1,157

Zone 3 Armor Layer Thickness = 0.41 m (16”) Filter Layer Thickness = 0.15 m (6”) 65 174

Backfill material was delivered to the site via truck and stored at the southeastern corner of the former food-processing facility parking lot area, just south of the Queen City Bridge. The on-site front-end loader then delivered the material to a transfer area constructed in the access ramp, which was lined with polyethylene sheeting and contained by jersey barriers. From there, a long-reach excavator located at the offloading area transferred the material to a three-compartment barge docked at the shoreline (as shown in Figure 5). The three-compartment barge, secured to the 500-horsepower hopper scow by chains and cables, was then pushed upstream to the active backfilling operations area.

Figure 5 - Loading backfill materials to the three-compartment barge.

Backfill material was placed over the entire dredged area to provide erosion protection similar to the protection that was previously provided by the visually impacted sediments. Backfill material was placed using the same

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equipment used to perform the site dredging, with the addition of the floating barge fitted with three metal compartments used to transport material from the loading area to the dredge prism.

The majority of the backfill material placement was verified and recorded using the HYPACK GPS system present on the excavator that was previously utilized during dredging operations. Verification was accomplished by placing the clamshell bucket on the river bottom to determine a pre-grade elevation and then evenly distributing backfill material throughout the area until the appropriate post-grade elevation was met.

A total of 6,330 tonnes (6,978 tons) of backfill material, as calculated from the material delivery weight tickets, was placed throughout the dredge prism. This volume equates to approximately 95 percent of the design tonnage. Of the 6,330 tonnes (6,978 tons) of backfill material placed, 4,150 tonnes (4,574 tons) were armor layer stone and 1,407 tonnes (1,551 tons) were filter layer material. Backfill depth placement verification was provided through a combination of bathymetric survey data, Contractor’s HYPACK data, and excavator boom marking information provided by the Contractor in areas with poor GPS coverage. The discrepancy between design tonnage and actual tonnage placed was most likely due to large pieces of metal and rock debris being left in place in the dredge prism and backfill being subsequently placed to grade around the debris. Other factors influencing backfill placement tonnage include sediment deposition from upriver and the reduction of dredge design cut depth in three polygons, resulting in smoother transitions between polygons.

Deviations from Backfilling Design

As previously detailed, encountered debris was too large in certain areas to be removed using the equipment on-site in which case it was characterized and left in place. During backfilling operations in these areas, backfill material was placed to the specified design depth around the debris, but not over the top of the obstruction. This deviation resulted in a slightly reduced volume of backfill being placed in those areas.

Another change to the backfilling design plan occurred in the two hardened tar deposit areas. The equipment utilized at the site was able to remove the hardened tar material that was located in the river; however, while dredging these areas, it was discovered that the impacted sediments extended outside of the dredging limits and into an area where sediment removal was restricted by the presence of protected vegetation. In order to not disturb this protected vegetation, an alternative remediation technique would need to be implemented to control the oil-like sheen emanating from the newly exposed shoreline sediments to the river. It was determined that placing an organoclay reactive mat would be the best course of action in these areas. Once placement approval from the permitting agencies was granted, the organoclay reactive matting was placed using the long-reach excavator operating from the dredge platform and aided by workers wading in shallow water near the shoreline. The matting was armored using the backfill material placed in adjacent areas of the river. The matting provided an immediate elimination of the oil-like sheen in these areas.

Similar to what was experienced during dredging operations, the HYPACK system present on the long-reach excavator was unable to gain adequate GPS satellite signal strength to accurately record material depth placement in areas beneath the overhead railroad bridge and in nearshore areas where overhanging trees were present. In these areas, 0.31 m (1 foot) interval boom markings placed on the excavator boom were utilized to determine when the designed backfill material placement depth had been met. The operator would begin by placing the long-reach excavator bucket on the river bottom, noting the boom marking, and then placing material until the bucket could be placed on top of the backfill and indicate that the appropriate depth had been met. Boom marking calculations in these areas were recorded in the appropriate field books accompanied by approximate locations and were later used to verify that designed backfill depths were achieved.

PROJECT CONCLUSIONS

The remedial action goals of the project were met by mechanically removing 6,048 m3 (7,910 cy) of material from the river. The dredge prism from which the material was removed was designed to remove the visually impacted surficial sediment located at the site. A total of 6,330 tonnes (6,978 tons) of backfill was placed over the dredge prism following dredging activities. The backfill placed was designed based on the distribution of the flow velocity and the 100-year flow depth as predicted by the hydrodynamic model. A total of 702.9 m3 (185,700 gallons) of water were treated on-site and discharged to the city sewer system over the course of the project.

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The site was restored to conditions similar to those existing prior to the start of construction activities with the exception of the paving of the removed section of the River Walk path. The paving has been suspended until spring 2008 due to weather constraints and material availability at the time of project completion.

REFERENCES

Anchor Environmental, L.L.C. (Anchor). (2008). Remedial Action Implementation Report, Merrimack River, Manchester, New Hampshire. March.

Anchor Environmental, L.L.C. (Anchor). (2007). Remedial Design Report, Merrimack River, Manchester, New Hampshire. May.

U.S. Army Corps of Engineers (Corps). (1991). Engineering and Design – Hydraulic Design of Flood Control Channels, EM 1110-2-1601, U.S. Government Printing Office, Washington D.C.

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